Multiple-Input Multiple-Output (MIMO) means a scheme that uses a plurality of transmitting antennas and a plurality of receiving antennas. Transmission and reception efficiency can be improved by the MIMO scheme. Namely, a transmitting side or receiving side of a wireless communication system can enhance capacity and improve throughput by using a plurality of antennas. Hereinafter, MIMO may be referred to as ‘MIMO antenna’.
The MIMO antenna technology does not depend on a signal antenna path to receive a whole message. Instead, in the MIMO antenna technology, data fragments received from a plurality of antennas are incorporated to complete data. If the MIMO antenna technology is used, a data transmission rate can be improved within a specific sized cell region, or system coverage can be enhanced with a specific data transmission rate. Also, the MIMO antenna technology can widely be used for a user equipment for mobile communication and a relay node. According to the MIMO antenna technology, it is possible to overcome limitation of a transmission rate in mobile communication according to the related art where a single antenna is used.
A schematic view of a general MIMO communication system is illustrated in FIG. 1. Referring to FIG. 1, NT number of transmitting antennas are provided at a transmitting side while NR number of receiving antennas are provided at a receiving side. If a plurality of antennas are used at both the transmitting side and the receiving side, theoretical channel transmission capacity is more increased than that a plurality of antennas are used at any one of the transmitting side and the receiving side. Increase of the channel transmission capacity is proportional to the number of antennas. Accordingly, the transmission rate is improved, and frequency efficiency is also improved. Supposing that a maximum transmission rate is Ro when a single antenna is used, a transmission rate corresponding to a case where multiple antennas are used can be increased theoretically, as expressed by the following Equation 1, as much as a value obtained by multiplying a maximum transmission rate Ro by a rate increase Ri. In this case, Ri corresponds to a smaller value of NT and NR.Ri=min(NT,NR)  [Equation 1]
For example, in a MIMO communication system that uses four transmitting antennas and four receiving antennas, a transmission rate four times greater than that of a single antenna system can be obtained. After such theoretical capacity increase of the MIMO system has been proved in the middle of 1990, various technologies have been actively studied to substantially improve a data transmission rate. Some of the technologies have been already reflected in the standard of various wireless communications such as third generation mobile communication and next generation wireless LAN.
Upon reviewing the recent trend of studies related to the MIMO system, active studies are ongoing in view of various aspects such as the study of information theoretical aspect related to MIMO communication capacity calculation under various channel environments and multiple access environments, the study of radio channel measurement and model of a MIMO system, and the study of time space signal processing technology for improvement of transmission reliability and transmission rate.
In order to describe a communication method in a MIMO system in more detail, mathematical modeling of the communication method can be expressed as follows. As illustrated in FIG. 1, it is assumed that NT number of transmitting antennas and NR number of receiving antennas exist. First of all, a transmitting signal will be described. If there exist NT number of transmitting antennas, since the number of maximum transmission information is NT, the transmission information can be expressed by a vector shown in Equation 2 as follows.s=└s1, s2, . . . , sNT┘T  [Equation 2]
In the mean time, different kinds of transmission power can be applied to each of the transmission information s1, s2, . . . , sNT. At this time, supposing that each transmission power is P1, P2, . . . , PNT, transmission information of which transmission power is controlled can be expressed by a vector shown in Equation 3 as follows.ŝ=[ŝ1, ŝ2, . . . , ŝNT]T=[P1s1, P2s2, . . . , PNTsNT]T  [Equation 3]
Also, ŝ^ can be expressed by Equation 4 below using a diagonal matrix P.
                              s          ^                =                                            [                                                                                          P                      1                                                                                                                                                                                                                                                                                0                                                                                                                                                                                                                P                      2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    ⋱                                                                                                                                                                                          0                                                                                                                                                                                                                                                                                  P                                              N                        τ                                                                                                        ]                        ⁡                          [                                                                                          s                      1                                                                                                                                  s                      2                                                                                                            ⋮                                                                                                              s                                              N                        T                                                                                                        ]                                =          Ps                                    [                  Equation          ⁢                                          ⁢          4                ]            
In the mean time, it is considered that a weight matrix W is applied to the information vector ŝ^ of which transmission power is controlled, so as to obtain NT transmitting signals x1, x2, . . . xNT. In this case, the weight matrix serves to properly distribute the transmission information to each antenna depending on a transmission channel status. Such transmitting signals x1, x2, . . . xNT can be expressed by Equation 5 below using a vector X. In this case, Wij means a weight value between the ith transmitting antenna and the jth information. W may be referred to as a weight matrix or precoding matrix.
                                                        x              =                            ⁢                              [                                                                                                    x                        1                                                                                                                                                x                        2                                                                                                                        ⋮                                                                                                                          x                        i                                                                                                                        ⋮                                                                                                                          x                                                  N                          T                                                                                                                    ]                                                                                        =                            ⁢                                                [                                                                                                              w                          11                                                                                                                      w                          12                                                                                            …                                                                                              w                                                      1                            ⁢                                                          N                              T                                                                                                                                                                                                                    w                          21                                                                                                                      w                          22                                                                                            …                                                                                              w                                                      2                            ⁢                                                          N                              T                                                                                                                                                                                          ⋮                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                            w                                                      i                            ⁢                                                                                                                  ⁢                            1                                                                                                                                                w                                                      i                            ⁢                                                                                                                  ⁢                            2                                                                                                                      …                                                                                              w                                                      iN                            T                                                                                                                                                              ⋮                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                            w                                                                                    N                              T                                                        ⁢                            1                                                                                                                                                w                                                                                    N                              T                                                        ⁢                            2                                                                                                                      …                                                                                              w                                                                                    N                              T                                                        ⁢                                                          N                              T                                                                                                                                                            ]                                ⁡                                  [                                                                                                                                          s                            ^                                                    1                                                                                                                                                                                          s                            ^                                                    2                                                                                                                                    ⋮                                                                                                                                                                  s                            ^                                                    J                                                                                                                                    ⋮                                                                                                                                                                  s                            ^                                                                                N                            T                                                                                                                                ]                                                                                                        =                            ⁢                              W                ⁢                                  s                  ^                                                                                                        =                            ⁢              WPs                                                          [                  Equation          ⁢                                          ⁢          5                ]            
Generally, a rank in the channel matrix may physically mean the maximum number of rows or columns that can transmit different kinds of information from a given channel. Accordingly, since a rank of the channel matrix is defined by a minimum number of independent rows or columns, it is not greater than the number of rows or columns. For example, a rank H of the channel matrix H is restricted as illustrated in Equation 6 below.rank(H)≦min(NT, NR)  [Equation 6]
Also, different kinds of information transmitted using the MIMO technology will be defined as ‘transport stream’ or more simply as ‘stream’. This stream may be referred to as a ‘layer’. In this case, the number of transport streams cannot be greater than the rank of the channel, which corresponds to the maximum number that can transmit different kinds of information. Accordingly, the channel matrix H can be expressed by the following Equation 7.#of streams ≦rank(H)≦min(NT, NR)  [Equation 7]
In this case, “# of streams” represents the number of streams. Meanwhile, it is to be understood that one stream can be transmitted through one or more antennas.
Various methods for corresponding one or more streams to several antennas may exist. These methods can be described, as follows, depending on the types of the MIMO technology. If one stream is transmitted through several antennas, it may be regarded as a spatial diversity scheme. If several streams are transmitted through several antennas, it may be regarded as a spatial multiplexing scheme. Of course, a hybrid scheme of the spatial diversity scheme and the spatial multiplexing scheme may exist.